Worst-case dollar cost of a few feet of rising seas could reach $100 trillion every year, according to the most rigorous scientific study of its kind.

Flooding caused by Hurricane Katrina in the New Orleans area in 2005. The global economic damages of failing to build dikes, levees, sea walls and other flood protections could be almost beyond comprehension as seas rise from global warming, a new scientific study has found. Credit: Paul Morse

The world needs to invest tens of billions of dollars a year in beefing up shoreline defenses against rising oceans or it will face mind-boggling costs in the decades to come, according to new research published this week in the Proceedings of the National Academy of Sciences.

If nations don't build up dikes, levees and sea walls, harden existing infrastructure, and preserve natural sponges like wetlands and barrier islands—and if they also do nothing to cut the emissions of greenhouse gases that cause global warming and are driving sea levels higher—the damages could be almost beyond comprehension, the researchers warned.

In a worst case, almost five percent of the world's population could be exposed to flooding at the start of the next century, and the damage could surpass nine percent of future global GDP each year.

This future damage from floods, they wrote, "may be one of the most costly aspects of climate change."

Rounded off, this worst-case dollar cost of rising sea levels could reach $100 trillion by the year 2100, they estimated. That's no typo: It's one hundred thousand billions.

Even taking into account the uncertainties in any such work, which the authors recognize, that is a pretty impressive cost to be paid for a few feet of rising seas. And its publication in such a prestigious peer-reviewed journal means it is not to be lightly shrugged off.

They called for a better understanding of the future costs of today's emissions, saying this is "essential for supporting efforts to reduce emissions driving sea-level rise as well as for designing strategies to adapt to increased coastal flood risk."

If their assessment holds up, global average storm surge damages, on a business-as-usual track, would have increased from between roughly $10 and $40 billion per year today to up to $100 trillion a year by the end of century.

The authors, from the Berlin-based think-tank Global Climate Forum and Britain's University of Southampton, said their study described for the first time the findings of a comprehensive global simulation of future flood damages to buildings and infrastructure in coastal flood plains.

He warned that "if we do not reduce greenhouse gases swiftly and substantially, some regions will have to seriously consider relocating significant numbers of people in the longer run."

What's most striking, though, is not the unfathomable figure they cited as their high-end cost. It is that by investing between $10 billion and $70 billion a year, the world could cut back the damages to a mere $80 billion or so a year in the early 22nd century.

Cutting dollar damages from rising seas from $100,000 billion to $80 billion a year would be a hearty return on investment, and well worth the expense, the authors argued.

Robert Nicholls of the University of Southampton, a co-author of the paper, said: "If we ignore sea-level rise, flood damages will progressively rise and presently good defenses will be degraded and ultimately overwhelmed. Hence we must start to adapt now, be that planning higher defenses, flood proofing buildings and strategically planning coastal land use."

"Poor countries and heavily impacted small-island states are not able to make the necessary investments alone, they need international support," he said.

But Nicholls questioned whether nations were capable of acting now to stave off devastation a hundred years from now.

"This long-term perspective is however a challenge to bring about, as coastal development tends to be dominated by short-term interests of, for example, real-estate and tourism companies, which prefer to build directly at the waterfront with little thought about the future," he said.